Absorption heat pumps, conventionally reserved for applications at low temperatures, such as refrigeration and air-conditioning, offer at the present time new opportunities with regard to the revalorization of energy in industry. They allow heat to be transferred from a low temperature (50-80.degree. C.) to a high temperature (100 to 150.degree. C.).
Water-alkali metal halide mixtures and, more particularly, water-lithium bromide mixtures, currently used in conventional absorption refrigeration systems, are quite compatible with such high transfer temperatures, provided that they are used at high concentrations by weight of salt (of the order of 60 to 70%) and at working temperatures which may attain 220.degree. C. Under these conditions of use, said mixtures are very aggressive towards the metallic materials of the system and the prevention of corrosion, becoming a major technological objective, requires new solutions which the teachings of the prior art cannot bring.
In fact, in conventional absorption machines, the water-lithium bromide mixtures do not function at temperatures higher than 160.degree. C. and their salt concentration does not exceed 60%. The risks of corrosion of the metallic materials, in contact with such mixtures, are limited by the use of corrosion inhibitors, such as lithium chromate; the molybdates, nitrates or nitrites of alkali metals, and in particular of lithium; benzotriazole and derivatives thereof; antimony trioxide; and certain mixtures thereof... Such aqueous mixtures and the corrosion inhibitors that they contain, as well as systems employing them as working fluid, are known and described in particular in U.S. Pat. Nos. 2 755 170 and 3 218 259 and in Japanese Pat. Nos. 5-97460, 53-25288 and 58-224187.
At high temperatures, said corrosion inhibitors according to the prior art sometimes present disadvantages which preclude or limit use thereof. For example, the polyalcohols are no longer stable; the chromates, nitrates, nitrites and molybdates are rapidly consumed and lead to a worsening of localized corrosion. The other inhibitors, which might be used under harsher conditions -- temperature close to 220.degree. C. and a salt concentration of the order of 70% -- have proved to be less effective.